About Mirroring and High Availability

A mirror provides data availability by replicating the data written to a virtual volume on a storage server to another virtual volume on a different storage server. Mirroring is the process of continuously updating data in real-time to maintain two or more identical copies of data on separate storage devices. Mirroring guards against data loss. To keep a remote copy of data that is consistent with the primary image, SANsymphony software uses “synchronous” mirroring over the SAN, in which the write acknowledgement is not returned to the requesting server until the data has been written to both storage servers. The mirrored virtual volumes stay in “lockstep.” Mirroring is generally viable with separations up to 100 kilometers. Synchronous mirroring typically takes place over dedicated high-throughput routes. The throughput of the channels used for I/O writes needs to be sufficient so as not to cause undue latency which would affect the responsiveness of servers.

A mirror is comprised of two (standard) or three (tertiary) virtual volumes. A standard mirror has a primary and secondary volume; these correspond to the storage servers that originated the volumes. The primary volume is from the primary server, which is based on the storage server you selected to provide the initial volume when creating the virtual volume. The other storage server by default becomes the secondary.

In addition to a standard mirror configuration, mirrors can be configured for high availability in different manners, including:

Alternate Path (AP)
3rd Party AP/MPIO (3PAP)
Cluster Path (CP)
Tertiary Mirrors

These configurations allow interruption-free access to data in the event of a disk or storage server failure. All of these product features operate independently of the hosts who generate the data stream and the underlying storage devices where their data is housed. They allow central and secure, remote mirroring services in a uniform manner, despite variations in operating systems and disk arrays.

High availability (HA), a system functioning with greater reliability than its individual components are capable, is most often achieved through fault tolerance. Fault tolerance in disk subsystems, is often achieved by including redundant instances of components whose failure would make the system inoperable, coupled with facilities that allow the redundant components to assume the function of the failed ones. Fault tolerant features include mirroring, RAID-5 volumes, and server clusters.

Alternate Pathing (AP) is an extension of mirrors handled by collaborating storage servers. It allows an application server interruption-free access to the virtual volume in the event of a component-volume failure or even the failure of a storage server. AP is able to redirect I/Os down a secondary path to the virtual volume when it detects a fault anywhere in the primary path to its primary volume. (DataCore Alternate Pathing (AP) software is at end-of-life, but is included for legacy purposes.)

Multi Path I/O (MPIO) is a high availability software feature that provides path redundancy to multi-path storage devices. More than two paths to virtual volumes are supported. DataCore’s support of MPIO simulates multi-pathing by means of multiple mappings of the same virtual volume. Refer to About DataCore MPIO for more information.

With Cluster Path (CP), the virtual volume is presented to two or more application servers; whereas with AP the virtual volume is presented to a single application server (except in the case of Cluster with AP). Additionally, the storage servers allow simultaneous access to the virtual volume via each of the paths. It is the responsibility of the cluster software running on the application servers to coordinate access to the shared virtual volume. The CP option is used in conjunction with cluster-enabled application servers, such as HACMP or Windows Clusters. Refer to About Cluster Path for more information.

Tertiary mirrors are, in effect, a duplicate image of a standard (two volume) mirror. They provide an additional layer of protection for critical information. At least three storage servers are required to create tertiary mirrors. Refer to About Tertiary Mirrors for more information.

Additionally, if there are multiple paths to a target, to provide mirror path failover, enable the Redundant Mirror Paths feature. Refer to Specifying the Mirror Path.

IMPORTANT: Mirror path configurations, such as using specific mirror paths or redundant mirror paths, must be configured prior to creating a mirror. After a mirror configuration exists for a virtual volume, mirror path settings cannot be changed. Refer to Specifying the Mirror Path to configure prior to creating mirrors.

Mirroring is a licensed optional feature. DataCore Alternate Pathing and MPIO software are also licensed optional features.

Informational topics related to mirroring include:

Sizing a Mirror

Mirror Discovery and Recovery

Best Practices for Mirroring

Best Practices for iSCSI High Availability

About Tertiary Mirrors

Procedural topics related to mirroring:

Creating mirrors:

Creating Mirrors for Virtual Volumes

Creating Mirrors Using Drag and Drop

Changing mirror attributes: